System principle

Zinc flake coating is a non-electrolytically applied corrosion-protection system based on metallic zinc/aluminium flakes in an inorganic/organic binder, with or without topcoat depending on specification. It is widely considered for fasteners, springs, clips, brackets and complex steel parts where hydrogen embrittlement risk from electrolytic processes must be reduced or avoided.

The coating should be treated as a system, not as a generic paint. Pretreatment, shot blasting, coating viscosity, dip-spin or spray route, draining, curing, coating mass, topcoat and packaging all influence corrosion performance and assembly behaviour.

Dip-spin versus spray route

Dip-spin is suitable for many bulk fasteners and smaller components where excess coating can be centrifuged away. It can deliver coverage on multiple orientations, but recesses, internal drives, blind holes and nested parts must be reviewed to avoid retained coating or stuck components.

Spray coating is useful for larger, delicate or geometry-sensitive parts where controlled application is required. Spray may be better when masking, one-sided control, appearance or local build-up must be managed. Route selection depends on geometry, quantity, part sensitivity and required approval.

Thickness, coating mass and curing

Zinc flake systems are commonly controlled by dry film thickness, coating mass and performance testing. A customer should specify whether the requirement is based on ISO 10683, customer standard, coating supplier TDS or an agreed SST target. Coating mass targets should not be copied blindly from one part to another because surface area, drainage and geometry affect actual deposit.

Curing is critical. Under-curing can reduce film integrity and chemical resistance; over-heating may affect appearance or substrate condition depending on part type. Parts should cool before dense packing to prevent condensation and imprinting.

Fitment, threads and recesses

Threads, bores, internal drives, blind holes and recesses need special discussion. Zinc flake can bridge, fill or retain in cavities if the part is unsuitable for the selected route. ISO 10683 explicitly addresses zinc flake fasteners and makes thread/forming and recess considerations relevant to system selection.

Where torque-tension, coefficient of friction or assembly performance is important, the customer should define lubricant/topcoat requirement and validate samples under actual assembly conditions. A corrosion-only approval is not enough for functional fasteners.

RFQ and approval checklist

  • Part drawing, material, heat treatment and whether the part is fastener, spring, clip, casting or fabrication.
  • Required standard: ISO 10683, customer specification or coating supplier requirement.
  • Route preference: dip-spin, spray, number of coats, topcoat and lubricant requirement.
  • SST target, friction requirement, masking, thread gauge and packaging expectation.
  • Customer validation for assembly, torque, fitment and actual exposure before bulk approval.